The disposal of chelate containing waste waters in a manner that minimizes deleterious environmental effects, and which meets the various governmental standards, can be a significant challenge to various industries. Such industries include the industries generating such waste waters and certain of the waste disposal industries.
A chelate, or chelation complex, is a complex of a compound that contains donor atoms, called a chelating agent, and a metal atom, combined by coordination bonding. Chelation is a means for manipulating and controlling metal ions; the chelation complexes usually have properties that are markedly different from their precursors. Chelation is used in many industries, including without limitation in scale removal from the water side of heat exchange equipment, boilers and the like, in water softening, in ore leaching, in textile processing and many other industries. Chelation may be used to reduce undesirable effects of metal ions, or to provide solubilization, or to provide metal buffering, or other effects.
Industrial chemical cleaning wastes often contain a significant concentration of chelates. Chelating agents are often included in the original cleaning solution, for instance for scale removal activity. The waste waters from such chemical cleaning operation thus often contains such a high level of chelates, or chelation complexes, that treatment of such wastes before disposal is desirable. Such wastes, or spent cleaning solutions, generally require treatment to separate the metal ions from the water, which generally entails freeing the metal ions from the chelates, preventing chelates from reforming, and at times destruction, or removal, of the chelating agents.
Industrial wastes other than spent cleaning solutions may also have significantly high levels or chelates, or may during processing become so concentrated that they then contain high levels of chelates. Such wastes may also require disruption of chelation activity so that the metals being held within the chelates may be removed.
Iron is one of the most prevalent metals in aqueous systems and hence one of the most prevalent targets for metal removal treatments. Iron itself can be removed from aqueous medium, as a precipitate of insoluble iron salt(s), particularly iron hydroxide salt(s), by aeration, pH elevation, flocculation, or combinations of such method. Iron has, however, a strong affinity for chelating agents, and thus conventional methods are generally ineffective in the presence of a significant level of chelating agents. As noted above, the chelates formed by the combination of metal and chelating agents must be broken and not permitted to reform.
Iron may not be the only metal of concern in industrial waste waters. Copper, chromium, nickel, zinc, lead, magnesium, barium, manganese, aluminum, cobalt, molybdenum, titanium, and others may be present in various concentrations and in various combinations. The removal of such metals also may be complicated or frustrated by their combination, or potential of combining, with chelating agents present in the waters.
Conventional methods used to break chelation complexes, and thus release metal ions held therein, generally require drastic elevations in pH, for instance elevations to pH's in excess of 9, and in some instances to pH's of 10 or 11, or even higher. Such highly alkaline conditions may themselves render the treated waste waters unsuitable for disposal or recycle or other uses. Further, the high pH might not break a sufficient amount of chelation complexes to allow the degree of metal precipitation required in order to meet discharge standards; in such instances the alkaline treated waste waters would also still be unsuitable for discharge.